EVOLUTION OF THK HORSE. 



The last in the evohitionary line is the genus Kquus, 

 which is first met with in the Upper Pliocene, an animal in 

 all essential details of structure similar to our modern horses. 



A, Orohippus (Eocene) ; H, Mesoliippus (Miocene) ; C, Protohippus 

 (Pliocene) : D, Equus (Pleistocene ;in(l Recent). (After Marsh) . 



Briefly reviewing this coordinating degeneration and devel- 

 opment in its relation to environment, there appears a remark- 

 able example of the power of the animal organism to keep 

 pace with changing surroundings in adapting itself to new 

 requirements. At the beginning of the Age of Mammals, 

 where we find the four-toed Eohippus, the North American 

 climate was tropical, and dense forests covered the greater 

 part of the country : the ground was moist, and there was an 

 abundance of tender, green food. The spreading, lateral 

 toes of the little primitive horses were well adapted to the soft 

 turf : their short, tuberculate teeth were sufficient for the 

 tender herbage, and the dense, tropical vegetation offered 

 them protection against their enemies. Horse and climate 

 now evolve together, the continent is steadily rising, the for- 

 ests become thinned, the land is getting drier, the climate 

 colder, and beasts of prey are becoming more formidable and 

 swifter of foot. The animals must adapt themselves to these 

 changed conditions or become extitict. The ancestors of the 

 horse take the former cotirse ; they become larger, the lateral 



